1. Field of the Invention
This invention relates generally to a two zone ranging system for a photographic camera apparatus and, more particularly, to a two zone active infrared ranging system for a photographic camera apparatus utilizing a low output, low cost infrared emitting diode.
2. Description of the Prior Art
Infrared ranging systems for photographic cameras are now well known in the art. One such infrared ranging system by Canon operates on the well-known principle of triangulation by beaming an infrared signal at the subject to be photographed and establishing the subject distance from the manner in which the infrared signal is reflected. This rangefinding system consists of an infrared emitting diode that is shifted sidewards during the measuring cycle and a fixed receiving cell. The traveling infrared emitting diode moves through a path of about ten millimeters and during this time emits a continuous signal in a specific wavelength band chosen to avoid interference from natural infrared sources such as sunlight. Also, during this time a sensing cell monitors the intensity of the infrared radiation reflected from the subject and converts this to an output voltage which is measured. When the output voltage reaches a peak and starts to decline, a control circuit stops the movement of the traveling infrared emitting diode and this movement is coupled with the movement of the objective lens which moves from a near to a far focusing limit such that the peak voltage signal can be utilized to determine the optimal focal position of the lens. This ranging system allows the objective lens to be set to an infinite number of focal positions corresponding to any camera to photographic subject distance determined by the detection of the peak voltage as the infrared emitting diode is shifted sidewards. Ranging systems of the above-described type are referred to as active systems in that they require both a source of radiance preferably in the infrared frequency range and a radiation sensor for sensing the reflected radiance from the photographic subject. In addition, either the source or the receptor must be displaced laterally in order to achieve the rangefinding function by the principle of triangulation.
Simple rangefinding systems which do not require the mechanical complexity required to provide for the lateral displacement of either the radiation source or the radiation detector are also well known in the art as taught in U.S. Pat. No. 4,357,083, entitled "Method and Apparatus Using Weighted Range Signal for Controlling Photographic Functions", by Bruce K. Johnson et al., issued Nov. 2, 1982, in common assignment herewith. The rangefinding system of the aforementioned Johnson et al. patent operates by directing a preliminary pulse of radiation from a selectively energizeable light source toward a subject and thereafter evaluating the amplitude of the reflected pulse of radiation from the subject so as to determine a weighted range signal as a function of the amplitude of generally non-visible or infrared radiation frequencies. This rangefinding system thus also provides an output ranging signal indicative of the actual position of the photographic subject within the field of view of the camera.
Such precision, however, in the determination of the camera-to-subject range may not always be necessary particularly for the design of a low cost camera in which it may be only necessary to provide two focal settings for the objective lens corresponding to a near camera-to-subject distance range and a far camera-to-subject distance range.
Therefore, it is a primary object of this invention to provide a low cost active rangefinding system which operates to provide an output signal indicative of whether the photographic subject is located in either a near distance zone from the photographic camera or a far distance zone from the photographic camera.
It is a further object of this invention to provide a low cost two zone rangefinding system which operates on the well-known principle of triangulation without requiring lateral movement of either the radiation source or the radiation detector.
It is an even further object of this invention to provide a low cost two zone active infrared rangefinding system which can utilize a low cost, low output infrared emitting diode for a source of radiation.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and system possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.